SQL Unique

What does the UNIQUE constraint do in SQL?

The UNIQUE constraint in SQL ensures that all values in a column are distinct. It prevents duplicate entries, maintaining data integrity and consistency. This is crucial for columns that represent unique identifiers or characteristics.

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Description

The UNIQUE constraint in SQL is a powerful tool for maintaining data integrity within a database. It ensures that no two rows in a table have identical values in a specified column. This is particularly important when dealing with columns that represent unique identifiers, like usernames, product codes, or customer IDs. By enforcing uniqueness, the UNIQUE constraint prevents data inconsistencies and errors that could arise from duplicate entries. This constraint is a fundamental aspect of relational database design, as it helps maintain the accuracy and reliability of the data stored within the database. For example, if you have a table of employees, a UNIQUE constraint on the employee ID column ensures that no two employees have the same ID. This constraint is enforced by the database management system (DBMS) and automatically rejects any attempt to insert a duplicate value into the column. It's important to note that the UNIQUE constraint differs from a PRIMARY KEY constraint, which also enforces uniqueness but also acts as the primary identifier for the table.

Why SQL Unique is important

UNIQUE constraints are vital for maintaining data accuracy and consistency in databases. They prevent data redundancy and ensure that each row in a table is uniquely identifiable. This is crucial for applications that rely on accurate data, such as e-commerce platforms, inventory management systems, and customer relationship management (CRM) systems.

SQL Unique Example Usage


-- Creating a temporary table
CREATE TABLE #temp_products (
    ProductID INT,
    ProductName VARCHAR(50),
    Price DECIMAL(10, 2)
);

-- Inserting data into the temporary table
INSERT INTO #temp_products (ProductID, ProductName, Price)
VALUES
(1, 'Laptop', 1200.00),
(2, 'Mouse', 25.00),
(3, 'Keyboard', 75.00);

-- Selecting data from the temporary table
SELECT * FROM #temp_products;

-- Calculating the total price of all products
SELECT SUM(Price) AS TotalPrice FROM #temp_products;

-- Dropping the temporary table (automatic when session ends)
DROP TABLE #temp_products;

Common Mistakes

Forgetting to define the UNIQUE constraint when it's necessary.
Using UNIQUE constraints on columns that are not intended to be unique.
Confusing UNIQUE constraints with PRIMARY KEY constraints.
Not understanding the implications of violating a UNIQUE constraint.

SQL Unique

What does the UNIQUE constraint do in SQL?

Description

Why SQL Unique is important

SQL Unique Example Usage

Common Mistakes

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